AbstractDuctility and robustness are two of the most important structural properties for the design of a building against extreme load cases, such as earthquakes and column loss scenarios. Various guidelines are based on the premise that an adequate amount of ductility is available and that structural robustness is attained. In this article, the outline of an experimental and analytical investigation related to the flexural behavior of a specific type of shallow (slim) floor beam, commonly known as Deltabeam, is presented. In order to determine the ductility and the robustness of the system, six full-scale composite beams with different geometry details were investigated under a three-point loading setup, up to large deflections. Tests showed that the ability of the concrete section, enclosing the steel beam, to withstand large strains without strength reduction strongly affected the ductility and robustness of the tested beams, which were thus far defined only by the class of their compressed steel components. The proper detailing of the reinforcement and the lateral protection provided by the stirrups were proven to be particularly important for the behavior of this type of beams when subjected to large deformations.